Nonresonant type knock sensor

ABSTRACT

A knock sensor comprises a sensor body having a metallic shell including a cylindrical portion and a flange portion formed at an end of the cylindrical portion, an annular piezoelectric element fitted around the cylindrical portion and an annular weighting member fitted around the cylindrical portion to hold the piezoelectric element between the weighting member and the flange portion, and a resin-molded sensor casing arranged circumferentially around the sensor body. The resin-molded sensor casing includes a weighting portion located nearer to the weighting member than to the piezoelectric element with respect to an axial direction of the cylindrical portion, and at least the weighting portion of the resin-molded sensor casing is made of a resin containing at least one of metal powder and metal oxide powder and has a density of 2.0 g/cm 3  or higher.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. application Ser. No.10/422,806 filed Apr. 25, 2003, the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a nonresonant type knock sensorthat detects the occurrence of knocking in an internal combustionengine.

[0003] A knock sensor with a piezoelectric element is commonly used inan ignition control system of an internal combustion engine so as todetect the occurrence of knocking in the engine and thereby allow thecontrol system to provide optimal ignition timing for the engine. Thereare two types of knock sensors: a resonant type and a nonresonant type.In the case of the nonresonant type knock sensor, the piezoelectricelement receives a mechanical load due to engine vibrations caused bythe knocking, converts the mechanical load into an electrical signal andoutputs the electrical signal to the control system via a band-passfilter so that the control system reads the signal output in a frequencyband corresponding to the knocking vibrations to find the occurrence ofknocking in the engine.

SUMMARY OF THE INVENTION

[0004] Recently, there have been strict environmental regulations. Whenthe piezoelectric element is made free from lead so as to be compliantwith such strict environmental regulations, there is a possibility thatthe signal outputted from the lead-free piezoelectric element is so weakthat the control system cannot determine whether the knocking isactually occurring in the engine. In order to avoid such a possibility,it is desired to improve the signal output characteristic of thenonresonant type knock sensor.

[0005] In consideration of the fact that the intensity of the outputsignal from the piezoelectric element depends on the mechanical loadapplied to the piezoelectric element, one conceivable way to improve thesignal output characteristic of the sensor would be to increase the sizeof any part or portion of the sensor that weights down the piezoelectricelement (such as a weighting member or resin-molded sensor casing) so asto add to its weight and thereby increase the mechanical load on thepiezoelectric element as disclosed in Japanese Laid-Open PatentPublication No. 2-173530. However, this results in upsizing of thesensor. As there is only a limited space for mounting the knock sensorin the engine, it is difficult to improve the signal outputcharacteristic of the sensor to a sufficient degree in theabove-mentioned way.

[0006] The present invention has been made allowing for theabove-mentioned circumstances, and an object of the present invention isto provide a nonresonant type knock sensor that has an increasedmechanical load on its piezoelectric element without upsizing of thesensor for improvement in signal output characteristic.

[0007] According to a first aspect of the invention, there is provided aknock sensor, comprising: a sensor body having: a metallic shellincluding a cylindrical portion and a flange portion formed at an end ofthe cylindrical portion; an annular piezoelectric element fitted aroundthe cylindrical portion; and an annular weighting member fitted aroundthe cylindrical portion to hold the piezoelectric element between theweighting member and the flange portion; and a resin-molded sensorcasing arranged circumferentially around the sensor body, wherein theresin-molded sensor casing includes a weighting portion located nearerto the weighting member than to the piezoelectric element with respectto an axial direction of the cylindrical portion, and at least theweighting portion of the resin-molded sensor casing is made of a resincontaining at least one of metal powder and metal oxide powder and has adensity of 2.0 g/cm³ or higher.

[0008] According to a second aspect of the invention, there is provideda knock sensor, comprising: a metallic shell including a cylindricalportion and a flange portion formed at an end of the cylindricalportion; an annular piezoelectric element fitted around the cylindricalportion; and an annular weighting member fitted around the cylindricalportion to hold the piezoelectric element between the weighting memberand the flange portion, wherein at least the flange portion of themetallic shell is made of a material having a lower specific gravitythan that of iron.

[0009] According to a third aspect of the invention, there is provided aknock sensor, comprising: a metallic shell including a cylindricalportion and a flange portion formed at an end of the cylindricalportion; an annular piezoelectric element fitted around the cylindricalportion; and an annular weighting member fitted around the cylindricalportion to hold the piezoelectric element between the weighting memberand the flange portion, wherein the flange portion has at least one cutformed therein to reduce the weight of the flange portion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a sectional view of a nonresonant type knock sensoraccording to a first or second embodiment of the present invention.

[0011]FIG. 2 is an exploded view of a sensor body of the knock sensor of

[0012]FIG. 3 is an illustration showing the operation of the nonresonanttype knock sensor.

[0013]FIG. 4 is a graph showing an improvement in signal output achievedby the knock sensor according to the second embodiment of the presentinvention under room temperature conditions.

[0014]FIG. 5 is a graph showing an improvement in signal output achievedby the knock sensor according to the second embodiment of the presentinvention under high temperature conditions.

[0015]FIG. 6 is a sectional view of a nonresonant type knock sensoraccording to a third embodiment of the present invention.

[0016]FIG. 7A is a sectional view of a metallic shell of the knocksensor of FIG. 6.

[0017]FIG. 7B is a bottom view of the metallic shell of FIG. 7A.

[0018]FIG. 8A is a sectional view of a metallic shell according to amodification of the third embodiment.

[0019]FIG. 8B is a bottom view of the metallic shell of FIG. 8A.

[0020]FIG. 9A is a sectional view of a metallic shell according toanother modification of the third embodiment.

[0021]FIG. 9B is a bottom view of the metallic shell of FIG. 9B.

DESCRIPTION OF THE EMBODIMENTS

[0022] The present invention will be described below with reference tothe drawings. In the following first to third embodiments, like partsand portions are designated by like reference numerals, and repeateddescriptions thereof are omitted.

[0023] A nonresonant type knock sensor 100 according to the firstembodiment of the invention will be first explained.

[0024] As shown in FIGS. 1 and 2, the knock sensor 100 comprises asensor body 190 having a metallic shell 120, an insulation sleeve 131,annular insulation plates 130 and 135, an annular piezoelectric element150, annular electrode plates 140 and 160, an annular weighting member170, a conical spring washer 180 and a nut 185, and a resin-moldedsensor casing 110.

[0025] The metallic shell 120 includes a cylindrical portion 121 and anannular flange portion 122 formed radially outwardly at an end 121 c ofthe cylindrical portion 121. The cylindrical portion 121 has a thread121 b formed on an outer circumferential surface thereof. Further, athrough hole 120 b is formed in the metallic shell 120 along an axialdirection of the cylindrical portion 121 in order for the knock sensor100 to be attached to a cylinder block of an internal combustion engine(not shown) by using a bolt (not shown) through the hole 120 b andthereby vibrate together with the cylinder block at the occurrence ofknocking. It is noted that the knock sensor 100 is mounted on thecylinder block in such an orientation that the flange portion 122 abutsat its bottom side on the cylinder block.

[0026] The insulation plate 130, the electrode plate 140, thepiezoelectric element 150, the electrode plate 160, the insulation plate135, the weighting member 170 and the spring washer 180 are fittedaround the cylindrical portion 121 of the metallic shell 120 in theorder of mention from the flange-portion side. The insulation sleeve 131is interposed between the cylindrical portion 121 of the metallic shell120 and the electrode plate 140, the piezoelectric element 150 and theelectrode plate 160 so as to keep the electrode plates 140 and 160 andthe piezoelectric element 150 electrically insulated from the metallicshell 120. The nut 185 has a thread 185 b formed on an innercircumferential surface thereof, and is screwed down against the springwasher 180 in such a manner as to fix the insulation plate 130, theelectrode plate 140, the piezoelectric element 150, the electrode plate160, the insulation plate 135 and the weighting member 170 between theflange portion 122 and the nut 185 by engagement of the threads 121 band 185 b. The electrode plates 140 and 160 has output terminals 141 and161, respectively, formed extendingly to output a signal from thepiezoelectric element 150 (i.e. a voltage developed between theelectrode plates 140 and 160) to an electronic control unit (ECU, notshown) via a band-pass filter (not shown).

[0027] The sensor casing 110 is arranged circumferentially around thesensor body 190 with the hole 120 b exposed externally of the sensorcasing 110. The sensor casing 110 includes a connector portion 113 inwhich the output terminals 141 and 161 are accommodated for connectionof the knock sensor 100 to the ECU. The sensor casing 110 furtherincludes a weighting portion 111 located nearer to the weighting member170 than to the piezoelectric element 150 with respect to the axialdirection of the cylindrical portion 121 of the metallic shell 120 tocontribute to the application of a load to the piezoelectric element150.

[0028] In the first embodiment, at least the weighting portion 111 ofthe sensor casing 110 is made of a resin containing at least one ofmetal powder and metal oxide powder and has a density of 2.0 g/cm³ orhigher at room temperature. The weighting portion 111 can be formedintegral with the other portions of the sensor casing 110 (the whole ofthe sensor casing 110 can be molded of the resin containing metal and/ormetal oxide powder). Alternatively, the weighting portion 111 may beformed separately from the other portions of the sensor casing 110 tohave e.g. a layer structure (only the weighting portion 111 may bemolded of the resin containing metal and/or metal oxide powder).

[0029] A resin-molded sensor casing of a conventional knock sensor isgenerally made of nylon and has a density of about 1.5 g/cm³, whereas atleast the weighting portion 111 of the sensor casing 110 is made of theresin containing metal and/or metal oxide powder and has a density of2.0 g/cm³ or higher as described above. Accordingly, the sensor casing110 becomes able to apply an increased mechanical load to thepiezoelectric element 150 even when the sensor casing 110 is made in thesame size as the above conventional sensor casing. This makes itpossible to improve the signal output characteristic of the knock sensor100 without upsizing of the sensor 100. This also makes it possible todownsize the knock sensor 100 while maintaining the signal outputcharacteristic of the sensor 100 at the same level as that of theconventional knock sensor.

[0030] Specific examples of the metal powder usable in the resin includetungsten powder, molybdenum powder, iron powder, stainless steel powderand the like. Specific examples of the metal oxide powder usable in theresin include tungstic oxide powder, molybdenum oxide powder, ferritepowder and the like. These metal and metal oxide powders can be usedalone or in any combination thereof.

[0031] The metal and/or metal oxide powder added in the resin preferablyhas a true density of 10.0 g/cm³ or higher at room temperature. If thevolume content of the metal and/or metal oxide powder in the resin isrelatively large, there is a possibility that the resin may becomedifficult to mold. When the metal and/or metal oxide powder has a truedensity of 10.0 g/cm³ or higher, however, it becomes possible to controlthe density of at least the weighting portion 111 of the metallic shell110 to 2.0 g/cm³ or higher without adding a large amount of the metaland/or metal oxide powder in the resin and thereby possible to avoid adeterioration in the moldability of the resin. Herein, the “truedensity” is defined as the density of a solid substance that formsparticles of the powder.

[0032] The metal and/or metal oxide powder added in the resin can beeither electrically conductive or insulative, but the sensor casing 110preferably has an insulating property in order to provide the insulationbetween axially opposite sides of the piezoelectric element 150 (e.g. tokeep the insulation resistance between the opposite sides of thepiezoelectric element 150 of 1 MΩ or higher) and to prevent theelectrode plates 140 and 160 from electrically conducting via the sensorcasing 110. In the case of the metal and/or metal oxide powder beingelectrically conductive, it is thus preferable to control the amount,particle size and particle shape of the metal and/or metal oxide powderadded. Especially when the sensor casing 110 is molded in one piece, itis desirable that the metal and/or metal oxide powder is electricallyinsulative so as to secure the insulating property of the sensor casing110 without regard to the amount, particle size and particle shape ofthe metal and/or metal oxide powder added in the resin. It becomestherefore possible to control the density of the resin-molding sensorcasing 110 to any desired value where the resin is moldable and adjustthe mechanical load on the piezoelectric element 150 as appropriate. Inparticular, the electrically insulative metal oxide powder (such astungstic oxide, molybdenum oxide and/or ferrite) is desirably used.

[0033] In consideration of effects on the human body, the metal and/ormetal oxide powder added in the resin is preferably free of lead.

[0034] As the resin of the sensor casing 110, a commercially availableresin, such as “MC102K07 (high-density resin with a density of 6.0g/cm³, prepared by adding tungsten powder to electrically insulativenylon 6)” from Kanebo., Ltd., can be used.

[0035] Further, the weighting member 170 preferably has a density of 10g/cm³ or higher at room temperature.

[0036] A weighting member of a conventional knock sensor is made of e.g.brass and has a density of about 8.0 g/cm³, whereas the weighting member170 has a density of 10 g/cm³ or higher. Accordingly, the weightingmember 170 becomes able to apply an increased mechanical load to thepiezoelectric element 150 even when the weighting member 170 is made inthe same size as the above conventional weighting member. This makes itpossible to improve the signal output characteristic of the knock sensor100 without upsizing of the sensor 100. In order to control the densityof the weighting member 170 to 10 g/cm³ or higher, the weighting member170 can be made of a heavy metal (such as tungsten or molybdenum), analloy thereof or a sintered metal thereof. In consideration of effectson the human body, the weighting member 170 is preferably free of lead.

[0037] Furthermore, the piezoelectric element 150 is desirably made of asintered piezoelectric ceramic material mainly composed of(Bi_(0.5)Na_(0.5))TiO₃, (Bi_(0.5)Ko_(0.5))TiO₃ and BaTiO₃ (hereinafterreferred to as “BNT”, “BKT” and “BT”, respectively).

[0038] Although the use of a lead-free piezoelectric element in a knocksensor being examined as an environmental protection measure, the knocksensor with the lead-free piezoelectric element generally shows a lowersignal output characteristic than that with a lead-containingpiezoelectric element as described above. With the piezoelectric element150 made of the BNT-BKT-BT sintered piezoelectric ceramic material to belead-free, however, it becomes possible for the knock sensor 100 toattain the signal output characteristic at the same level as that withthe lead-containing piezoelectric element. Herein, the term “lead-freepiezoelectric element” means a piezoelectric element containing lead inan amount of less than 0.001% by mass, as measured by fluorescent X-rayanalysis, based on the total mass of the piezoelectric element.

[0039] It is assumed that the chemical composition of the mainBNT-BKT-BT constituent of the sintered piezoelectric ceramic material isexpressed as BNT_(x)BKT_(y)BT_(z) where x, y and z (x+y+z=1) representthe mole fractions of the BNT, BKT and BT components, respectively. Inorder for the piezoelectric element 150 to attain high sensitivity andheat-resistance, it is desirable to control the mole fractions of theBNT, BKT and BT components in such a manner as to satisfy the followingexpressions: 0.5≦X≦0.9, 0≦y≦0.5 and 0≦z≦0.5. This allows the knocksensor 100 to show high sensitivity and heat resistance.

[0040] Next, a nonresonant-type knock sensor 200 according to the secondembodiment of the invention will be explained. The knock sensor 200 isstructurally similar to the knock sensor 100 as shown in FIG. 1, exceptthat at least of a flange portion 222 of a metallic shell 220 of theknock sensor 200 is made of a material having a lower specific gravitythan that of iron. The flange portion 222 can be formed integral with acylindrical portion 221 of the metallic shell 220 (the whole of themetallic shell 220 can be formed from the material having a lowerspecific gravity than that of iron). Alternatively, the cylindricalportion 221 and the flange portion 222 can be formed separately andjoined together by e.g. adhesive bonding or welding (only the flangeportion 222 can be formed from the material having a lower specificgravity than that of iron).

[0041] The operation of the knock sensor 200 will be now described belowwith reference to FIG. 3 in order to facilitate the understanding of thesecond embodiment. Herein, the effect of a sensor casing 210 is left outof consideration. When the knock sensor 200 receives an acceleration Awith the operation of the engine, the piezoelectric element 150 receivesa mechanical load F that can be expressed as the difference between aforce Ft acting on the weighting member 170 and a force Fs acting on theflange portion 222 (F=Ft−Fs). Then, the piezoelectric element 150develops a voltage output V responsive to the mechanical load F exertedon the piezoelectric element 150. As the forces Ft and Fs acting on theweighting member 170 and the flange portion 222 are proportional to aweight Wt of the weighting member 170 and a weight Ws of the flangeportion 222, respectively, it is concluded that the voltage output Vfrom the piezoelectric element 150 is in proportion to the differencebetween the weight Wt of the weighting member 170 and the weight Ws ofthe flange portion 222 (V∝Wt−Ws). Accordingly, the signal output fromthe piezoelectric sensor 150 can be increased with decrease in theweight of the flange portion 222.

[0042] A metallic shell of a conventional knock sensor is generally madeof iron or brass, whereas at least the flange portion 222 of themetallic shell 220 is made of the material having a lower specificgravity than that of iron. The flange portion 222 is therefore madelighter in weight so that the signal output characteristic of the knocksensor 200 can be improved without upsizing of the sensor 200 asdescribed above.

[0043] The material having a lower specific gravity than that of ironcan be exemplified by a resinous material (such as polyphenylenesulfide: PPS) and a metallic material. In consideration of heatresistance, it is desirable to use the metallic material, preferablyaluminum. The specific gravity of aluminum (about 2.7) is as low as onlyabout 35% of the specific gravity of iron (about 7.9). The use ofaluminum thus offers sufficient weight reduction of the flange portion222 for improvement of the signal output characteristic of the knocksensor 200. Further, aluminum is suitable for the metallic shell 222because of its hardness and availability. In addition, aluminum ishighly resistant to corrosion. Although the metallic shell made of ironneeds to be given plating (such as zinc chromate plating) so as toimprove corrosion resistance, such plating becomes unnecessary throughthe use of aluminum. It becomes possible to simplify the manufacturingprocess of the knock sensor 200.

[0044] A nonresonant-type knock sensor 300 according to the thirdembodiment of the invention will be described. The knock sensor 300 isstructurally similar to the knock sensors 100 and 200 as shown in FIG.6, except that a metallic shell 320 of the knock sensor 300 has at leastone cut formed in its flange portion 322 so that the flange portion 322can be made lighter in weight. This makes it possible to improve thesignal output characteristic of the knock sensor 300 without upsizing ofthe sensor 300 for the same reason as described above in the secondembodiment.

[0045] There may be edges and burrs caused by forming the cut or cuts inthe flange portion 322. In such a case, it is desirable that such edgesand burrs are given chamfering so that the flange portion 322 is closelyheld onto the piezoelectric element 150 and the cylinder block forstable signal output characteristic of the knock sensor 300.

[0046] The cut or cuts are preferably formed in one side of the flangeportion 322 opposite to the side facing toward the piezoelectric element150. If the cut or cuts are formed in the side of the flange portion 322facing toward the piezoelectric element 150, the piezoelectric element150 becomes less prone to vibrations caused by the knocking.Accordingly, there arises a possibility that the output voltage of thepiezoelectric element 150 may be lowered and/or the waveform of theoutput voltage of the piezoelectric element 150 may be distorted. Withthe cut or cuts formed in the side of the flange portion 322 opposite tothe side facing toward the piezoelectric element 150, however, itbecomes possible to effectively prevent the output voltage of thepiezoelectric element 150 from being lowered or distorted and, at thesame time, to reduce the weight of the flange portion 322 forimprovement in the signal output characteristic of the knock sensor 300.

[0047] As shown in FIGS. 7A and 7B, a single cut groove 322 d may beformed around a cylindrical portion 321 of the metallic shell 320 inorder to improve the signal output characteristic of the knock sensor300 effectively by reducing the weight of the flange portion 322 whilekeeping the weight balance of the flange portion 322. The groove 322 dcan be of any form, such as cyclic, star or polygonal form.Alternatively, a plurality of circumferentially evenly spaceddepressions 322 e may be formed around the cylindrical portion 321 asshown in FIGS. 8A and 8B. The shape of the depressions 322 e is notlimited to round shape, and can be any other shape, such as star orpolygonal shape. As shown in FIGS. 9A and 9B, a plurality of grooves 322f may be formed around the cylindrical portion 322 f. In such cases, itis also possible to use the depressions 322 e or 322 f for e.g. thefixing and positioning of the metallic shell 320 during the assembly ofthe knock sensor 300 in addition to reducing the weight of the flangeportion 322.

[0048] Instead of forming at least one cut in the flange portion 322,one side of the flange portion 322 can be cut away in such a manner asto reduce the thickness of the flange portion 322 and thereby reduce theweight of the flange portion 322.

[0049] Further, the metallic shell 320 preferably has at least theflange portion 322 made of the material having a lower specific gravitythan that of iron, more preferably aluminum, in the same manner as inthe second embodiment to further reduce the weight of the flange portion322.

[0050] The present invention will be described in more detail byreference to the following examples. It should be however noted that thefollowing examples are only illustrative and not intended to limit theinvention thereto.

EXAMPLES

[0051] Various samples of knock sensors were manufactured and tested forperformance as follows.

[0052] A sample of the knock-sensor 100 (SAMPLE 1) was manufactured bythe following procedure. The respective sensor body parts were firstprepared using the following materials: soft iron for the metallic shell120 and the nut 185; polyolefin for the insulation sleeve 131;polyethylene terephthalate (PET) for the insulation plates 130 and 135;42Ni—Fe alloy for the electrode plates 140 and 160; lead zirconatetitanate (PZT) for the piezoelectric element 150; and tungsten (density:about 19.2 g/cm³) for the weighting member 170. The prepared body partswere assembled into the sensor body 190, by: putting the insulationsleeve 131 on the cylindrical portion 121 of the metallic shell 120;fitting the insulation plate 130, the electrode plate 140, thepiezoelectric element 150, the electrode plate 160 and the insulationplate 135 around the insulation sleeve 131 in the order of mention;placing the weighting member 170 on the insulation plate 135 to hold thepiezoelectric element 150, the insulation plates 130 and 135 and theelectrode plates 140 and 160 between the weighting member 170 and theflange portion 122; putting the spring washer 180 on the weightingmember 170; and then screwing the nut 185 against the washer 180 in sucha manner as to hold the insulation plates 130 and 135, the electrodeplates 140 and 160, the piezoelectric element 150, the weighting member170 and the washer 180 between the flange portion 122 and the nut 185with a predetermined load imposed on the piezoelectric element 150.Then, a resin was prepared by mixing tungsten powder (true density:about 19.2 g/cm³) into nylon in such a manner that the density of theresin was controlled to about 2.1 g/cm³. The sensor casing 110 wasintegrally molded of the prepared tungsten-powder containing nylon resinby injection molding according to a known molding method, so as tocircumferentially surround the sensor body 190 with the hole 120 b ofthe metallic shell 120 exposed externally of the sensor casing 110.

[0053] For reference purposes, a knock sensor was prepared as REFERENCESAMPLE by the same procedure and with the same dimensions as used forSAMPLE 1, except that the corresponding weighting member and sensorcasing were made of brass (density: about 8.0 g/cm³) and nylon (density:about 1.5 g/cm³), respectively. The metallic shell of REFERENCE SAMPLEhad no groove/depression formed in its flange portion for weightreduction of the flange portion.

[0054] Performance comparisons were made between SAMPLE 1 and REFERENCESAMPLE. The weighting portion 111 of SAMPLE 1 had a density of about 2.1g/cm³ that was larger than that of the corresponding portion ofREFERENCE SAMPLE (about 1.5 g/cm³), so that the weighting portion 111 ofSAMPLE 1 weighed more than the corresponding portion of REFERENCE SAMPLEeven in the same size. The weighting member 170 of SAMPLE 1 had adensity of about 19.2 g/cm³ that was larger than that of thecorresponding member of REFERENCE SAMPLE (about 8.0 g/cm³), so that theweighting portion 170 weighed more than the corresponding member ofREFERENCE SAMPLE even in the same size. SAMPLE 1 was therefore able toapply an increased mechanical load to the piezoelectric element 150under the load of the weighting portion 111 and the weighting member 170without increasing in size and then to achieve an improved signal outputcharacteristic.

[0055] Further, the density of the resin of the sensor casing 110 wascontrolled to about 2.1 g/cm³ by adding a very small amount of thetungsten powder with a true density of about 19.2 g/cm³. The volumecontent of the tungsten powder in the resin was so low that the resinwas molded into the sensor casing 100 without trouble and did not causedeterioration in the insulation resistance between the opposite sides ofthe piezoelectric element 150.

[0056] Another sample of the knock sensor 100 (SAMPLE 2) wasmanufactured by the same procedure and with the same dimensions as usedfor SAMPLE 1, except that the electrically insulative tungstic oxide(WO₃) powder was used in place of the tungsten powder.

[0057] As compared to REFERENCE SAMPLE mentioned above, SAMPLE 2 wasable to attain an improved signal output characteristic in the samemanner as SAMPLE 1. In addition, the insulation property of the sensorcasing 110 was secured assuredly by the use of the electricallyinsulative tungstic oxide powder. There was no fear of electricalconduction between the electrode plates 140 and 160 via the sensorcasing 110 and no fear of insufficient insulation of the connectorportion 113. The density of the sensor casing 110 was controlled asappropriate without regard to the amount of the tungstic oxide powderadded to apply an increased mechanical load to the piezoelectric element150, while the moldability of the resin was maintained.

[0058] Next, a sample of the knock sensor 200 (SAMPLE 3) wasmanufactured by the same procedure and with the same dimensions as usedfor SAMPLE 1, except that the metallic shell 220, the weighting member170 and the sensor casing 210 were made of aluminum (available as “KS27”from Furukawa Electric Co., Ltd. according to JIS H4040), brass andnylon, respectively. In other words, SAMPLE 3 differed from REFERENCESAMPLE in that: the corresponding metallic shell of REFERENCE SAMPLE wasmade of iron, whereas the metallic shell 220 of SAMPLE 3 was made ofaluminum to reduce the weight of the flange portion 222.

[0059] Performance comparisons were made between SAMPLE 3 and REFERENCESAMPLE as follows. Each of SAMPLE 3 and REFERENCE SAMPLE was mounted inthe cylinder head of an internal combustion engine, and the signaloutputs from SAMPLE 3 and REFERENCE SAMPLE were measured at roomtemperature with respect to varying engine vibration frequencies. Theaverage of the measured signal outputs was calculated against eachvibration frequency. Then, the signal output ratio of SAMPLE 3 toREFERENCE SAMPLE at room temperature were calculated by the followingexpression:

Output ratio=(Al_(avg)−Fe_(avg))×100/Fe_(avg)

[0060] where Al_(avg) is the average of the signal outputs from SAMPLE 3at a given engine vibration frequency; and Fe_(avg) is the average ofthe signal outputs from REFERENCE SAMPLE at the given engine vibrationfrequency. The results are shown in FIG. 4. Further, the signal outputratio of SAMPLE 3 to REFERENCE SAMPLE was determined at 125° C. in thesame way as above. The results are shown in FIG. 5. As is apparent fromFIGS. 4 and 5, SAMPLE 3 had 15% or more of improvement in signal outputat room temperature and 23% or more of improvement in signal output at125° C. as compared to REFERENCE SAMPLE. SAMPLE 3 was able to apply anincreased mechanical load to the piezoelectric element 150 by reducingthe flange portion 222 in weight without increasing in size, andtherefore able to attain an improved signal output characteristic.Further, aluminum was suitably used for the metallic shell 220 due toits hardness and availability. There was no need to give platingtreatment in the preparation of the metallic shell 222 because of highcorrosion resistance of aluminum, so that the manufacturing process ofSAMPLE 3 was simplified.

[0061] A sample of the knock sensor 300 (SAMPLE 4) was manufactured bythe same procedure and with the same dimensions as used for SAMPLE 1,except that the weighting member 170 and a sensor casing 310 were madeof brass and nylon, respectively, and that the metallic shell 320 had agroove 322 d formed in one side of the flange portion 322 opposite tothe side facing toward the piezoelectric element 150. In other words,SAMPLE 4 differed from Reference example in that: the correspondingportion of REFERENCE SAMPLE had no groove, whereas the flange portion322 of the SAMPLE 4 had the groove 322 d formed therein to reduce theweight of the flange portion 322.

[0062] As compared to REFERENCE SAMPLE, SAMPLE 4 was able to apply anincreased mechanical load on the piezoelectric element 150 by reducingthe flange portion 322 in weight without increasing in size, andtherefore able to attain an improved signal output characteristic.

[0063] Another sample of the knock sensor (SAMPLE 5) was manufactured bythe same procedure and with the same dimensions as used for SAMPLE 4,except that the metallic shell 320 was made of aluminum so as to furtherreduce the weight of the flange portion 322. Accordingly, SAMPLE 5 wasable to attain more improvement in the signal output characteristic thanthat attained by SAMPLE 4.

[0064] Still another sample of the knock sensor 300 (SAMPLE 6) was bythe same procedure and with the same dimensions as used for SAMPLE 5,expect that the piezoelectric element 150 was made of a sinteredpiezoelectric ceramic material mainly composed of BNT, BKT and BT asfollows. The BNT-BKT-BT sintered piezoelectric ceramic material wasprepared by using as starting materials BaCO₃ powder, K₂CO₃ powder,Na₂CO₃ powder and TiO₂ powder. The BaCO₃ powder, K₂CO₃ powder, Na₂CO₃powder and TiO₂ powder were dispensed so that the ratio of molefractions x, y and z of BZT, BKT and BT components in the ceramicmaterial was controlled to x:y:z=0.80:0.10:0.10. Ethanol was added tothe BaCO₃ powder, K₂CO₃ powder, Na₂CO₃ powder and TiO₂ powder andsubjected to wet blending for 15 hours by using a ball mill. Theresultant mixture was put in hot water, dried, and calcinated at 800° C.for 2 hours. The calcinated mixture was subjected to wet milling for 15hours by using a boll mill, put in hot water and then dried to obtain agranulation of the BNT-BKT-BT sintered piezoelectric ceramic material.The granulation was formed to a predetermined size by uniaxial pressingwith a pressure of 1 GPa and subjected to cold isostatical press (CIP)with a pressure of 15 GPa. The thus-obtained formed article was sinteredat 1050 to 1250° C. for 2 hours. Silver electrodes were formed on thesintered article and subjected to polarization process, therebycompleting the piezoelectric element 150.

[0065] Although the piezoelectric element 150 of SAMPLE 6 was lead-free,SAMPLE 6 was able to attain the same level of signal outputcharacteristic as that of SAMPLE 5. As there was no dispersion of leadduring the sintering of the ceramic material, SAMPLE 6 was moreenvironmentally friendly. Further, the piezoelectric element 150 ofSAMPLE 6 satisfied the following expressions: 0.5≦X≦0.9, 0≦y≦0.5 and0≦z≦0.5 so that the piezoelectric element 150 had high sensitivity andheat resistance. Namely, SAMPLE 6 showed high sensitivity and heatresistance.

[0066] The entire contents of Japanese Patent Application Nos.2002-127301 (filed on Apr. 26, 2002), 2002-243746 (filed on Aug. 23,2002) and 2002-251320 (filed on Aug. 29, 2002) are herein incorporatedby reference.

[0067] Although the present invention has been described with referenceto specific embodiments of the invention, the invention is not limitedto the above-described embodiments. Various modification and variationof the embodiment described above will occur to those skilled in the artin light of the above teaching. For example, the weighting member 170,the conical spring washer 180 and the nut 185 may be formed into onepiece so as to reduce the parts count of the sensor. The scope of theinvention is defined with reference to the following claims.

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 9. (canceled).10. (canceled).
 11. A knock sensor, comprising: a metallic shellincluding a cylindrical portion and a flange portion formed at an end ofthe cylindrical portion; an annular piezoelectric element fitted aroundthe cylindrical portion; and an annular weighting member fitted aroundthe cylindrical portion to hold the piezoelectric element between theweighting member and the flange portion, wherein the flange portion hasat least one cut formed therein to reduce the weight of the flangeportion.
 12. The knock sensor according to claim 11, wherein said atleast one cut is formed in a side of the flange portion opposite to aside facing toward the piezoelectric element.
 13. The knock sensoraccording to claim 11, wherein said at least one cut is a groove formedaround the cylindrical portion.
 14. The knock sensor according to claim11, wherein said at least one cut includes a plurality of depressions.15. The knock sensor according to claim 11, wherein at least the flangeportion of the metallic shell is made of a material having a lowerspecific gravity than that of iron.
 16. The knock sensor according toclaim 11, wherein the piezoelectric element is made of a sinteredpiezoelectric ceramic material mainly composed of(Bi_(0.5)Na_(0.5))TiO₃, (Bi_(0.5)K_(0.5))TiO₃ and BaTiO₃.